Method of calibrating an x-ray detector

ABSTRACT

The general field of the invention includes methods of calibrating X-ray detection systems, the systems including at least one X-ray generator and a detection array having a matrix of detecting semiconductor pixels and processing and calibration electronics. The calibration method includes, for all or some of the pixels: operating the X-ray generator at its nominal high voltage, the generator being placed opposite the detector; counting, using the processing and calibration electronics, the pulses emitted by each pixel through the effect of the radiation produced by the generator; establishing, for each pixel, an amplitude distribution for the counted pulses; applying, to each amplitude distribution, a statistical indicator so as to identify a particular amplitude, this particular amplitude then corresponding to the energy corresponding to said statistical indicator; and adjusting, using the processing and calibration electronics, calibration parameters for each pixel, taking account of the energy-amplitude relationship thus established.

The field of the invention is that of photon-counting, matrix X-raydetectors. More precisely, the invention relates to a method ofcalibrating the pixels of the matrix forming the detector.

Monolithic semiconductor X-ray detectors comprise, as illustrated inFIG. 1, a planar substrate 1 made of a semiconductor. The detectormaterial may be from the family of detectors that are semiconductors atroom temperature. Mention will be made, notably, of CdTe, CdZnTe, GaAs,TIBr, Hgl₂ or CdMnTe.

This substrate 1 comprises, on one of its sides, a matrix of first,positively biased electrodes 2 and, on the opposite side, a matrix ofnegatively biased electrodes 3. Each pixel thus consists of anelementary anode connected to its processing electronics. The typicalpixel size, defined as the distance over which the electrodes repeat,may range from a few tens of microns to a few hundreds of microns.

When a photon X, the energy of which is located in the energy band froma few keV to a few hundred keV, crosses a semiconductor pixel it createsa number of electron-hole pairs 5 by ionizing atoms in the semiconductorcrystal. These charges are captured by the electrodes of the pixel andgenerate, during their transit, electrical pulses at these electrodes.These pulses are counted by an ASIC 4—“ASIC” standing for applicationspecific integrated circuit. The material 6 used to interconnect thesubstrate and the ASIC depends on the pixel size. Advantageously, indiumor any metal having a low melting point, typically less than 150° C., isused for small sizes or conducting polymer adhesives are used for largersizes.

For a radiation detection matrix using the counting principle, the pulseemitted by the elementary pixel is amplified, then compared to athreshold in order to decide whether the pulse is counted or not. Theamplitude of this threshold defines the pulse amplitude and thereforetheoretically the energy delivered to the detector by the incidentphoton. In the case of a multi-energy counting system, the variousthresholds are adjusted so that they each correspond to a preciseenergy. Thus, energy bands and images corresponding to these bands aredefined. These images are notably used in devices for imaging contrastagents or various tissues or for detecting explosives in the case ofluggage inspection.

For a matrix of a few thousand pixels, the response sensitivity of thecounting system as a function of the position of this threshold may varyquite significantly from one pixel to another, leading to responseinhomogeneities. Various methods are commonly used to adjust thesethresholds.

For example, to compensate for a temperature drift in the countingelectronics, a calibrated charge is injected into all the pixels and theadjustment is carried out by sweeping their thresholds. The thresholdsare then adjusted so that finally all the pixels only retain pulseshaving an amplitude which exceeds that of the injected signal. Thisinjection may be carried out at the pixel level by a dedicatedelectronic device. It may also be carried out via the capacitor C of thesemiconductor detector connected to each pixel. It may, for example, bea CdTe detector connected by an indium bump to a counting ASIC. Thesemiconductor detector then consists of a bias electrode on the top sideand on the bottom side of many electrodes connected to the readout ASIC,pixel by pixel. By applying a rapid voltage variation δV to the commontop electrode, an electrical pulse is created, the amplitude of which isC. δV/δt, in each small electrode connected to the input of theamplifier of the counting ASIC. One embodiment is presented in FIG. 2.The series of electronics located at the output of the pixel 10comprises:

a capacitor 11 enabling an amount of charge to be injected into theamplifier channel so as to calibrate the system;

a first amplification stage 12 that amplifies the charge packet comingfrom the detector 10 by making, in the case of the figure, acurrent-voltage conversion;

electronic means 13 enabling the signals to be temporally shaped;

electronic means 14 for defining, with an analog/digital converter, avoltage threshold, used by the comparator;

electronic means 15 for comparing the input voltage with the thresholdvoltage and delivering a logic signal when this input voltage is greaterthan the threshold; and

electronic means 16 for counting logic pulses in a given time intervaland then transmitting the result to a readout bus.

The device operates as follows. A voltage of a few volts is appliedupstream of the amplification stage 12 located at the output of thepixel 10 for a few nanoseconds to a few tens of nanoseconds, thussimulating the signal produced by an interaction of a photon of acertain energy in the detector. Next, the amplitude of the signaldelivered by the various means making up the readout electronics of thepixel is determined, resulting in the amplitude corresponding to thepulse “injected” upstream of the pixel. By carrying out this operationfor all the pixels of the detector, it is possible to know, for eachpixel, the amplitude corresponding to this given pulse. This amplitudeis then considered to be the threshold of each pixel. A detector, inwhich each pixel has been “thresholded”, is then obtained, eachthreshold corresponding to the same pulse and therefore to the sameenergy deposited in the detector.

However, this calibration method has certain drawbacks since thecorrelation between the energy deposited in the detector and theduration and intensity of the pulse is difficult to establish. Inaddition, such a method does not take into account differences in chargetransport and collection between each pixel. Indeed, two pixels maypossess identical electronic capacities but completely different chargetransport properties. In addition, the electronic circuits, and inparticular the readout circuits, are subject to thermal drift. Athreshold corresponding to a given energy may correspond, after acertain time, to a different energy. This relatively tedious operationmust therefore be repeated over the course of time.

To carry out a more reliable calibration, it is necessary to use anX-ray or gamma-ray source of known characteristics. In order for theamplitude of the pulses produced by a detector to correspond to thecorresponding energy of the incident photons, it is possible tocalibrate the system with monoenergetic radiation sources of knownenergy. These sources are generally not very active and the statisticsare insufficient even with long acquisition times. In the case of anX-ray tube, the energies emitted present an energy continuum between aminimum energy defined by the filtration of the generator, about 10 to20 keV, depending on the application, and a maximum energy defined bythe high voltage of the X-ray generator. It is possible to use themaximum energy as reference for the calibration. However, this is noteasy when the thresholds must be adjusted to low-energy values, between10 and 40 keV for example, since the number of photons emitted by thegenerator in this energy range is then very small. In addition, thegenerators used are not designed to emit at voltages located in therange from 10 to 40 kV. This is the case for example for X-ray scanners.Another method, described in the patent U.S. Pat. No. 7,479,639,consists in using radiation sources of well-known energy to calibratethe thresholds. However, this method is long and requires radiationsources to be present in the vicinity of the radiological system. Thequestion of managing these sources in order to meet various safetystandards then arises, all the more so in that the most active sourcespossible must be used if it is desired to achieve a correct calibrationin a reasonable period of time.

International patent application WO 2009/122317 provides a calibrationdevice using the radiation source of the detection system. However, thisdevice requires a material of known spectral radiance in order to carryout the calibration. Attention is drawn, in particular, to FIG. 1 in theabove application.

The method according to the invention does not have these drawbacks. Theproposed calibration method uses the X-ray generator present in X-raydetection systems. Thus, both the drawbacks of a purely electroniccalibration, which does not allow the entire detection channel to becalibrated, and the drawbacks of a calibration comprising standardsources or standard materials, necessarily more problematic toimplement, are avoided at the same time. For each pixel, an amplitudethreshold corresponding to the same energy is obtained by a simple andrapid method that takes into account the charge transport and collectionin the detector material. Advantageously, the method makes use of meanspresent at the point of use of the detector.

More precisely, the subject of the invention is a method of calibratingan X-ray detection system, said system comprising at least one X-raygenerator and a detection array comprising a matrix of detectingsemiconductor pixels and processing and calibration electronics,characterized in that the calibration method comprises, for all or someof the pixels, the various, following steps:

operating the X-ray generator at its nominal high voltage, the generatorbeing placed opposite the detector;

counting, using the processing and calibration electronics, the pulsesemitted by each pixel through the effect of the radiation produced bythe generator;

establishing, in each pixel, an amplitude distribution of the countedpulses;

applying, to each amplitude distribution, a statistical indicator so asto identify a particular amplitude, this particular amplitudecorresponding to an energy associated with said statistical indicator;and

adjusting, using the processing and calibration electronics, calibrationparameters for each pixel, taking account of the energy-amplituderelationship thus established.

Advantageously, the statistical indicator relating to said distributioncomprises the median or any other percentile, or the mean. The finalstatistical indicator may also be a combination of various statisticalindicators.

According to a first embodiment, in each pixel, the energy valuecorresponding to the amplitude of the signal corresponding to saidindicator is treated as being an energy threshold.

According to a second embodiment, in each pixel, the pulses having anamplitude below a given threshold are not processed by the readoutelectronics of the pixel.

According to a third embodiment, such a method allows, for all thepixels, at least one energy window, lying between a first threshold anda second threshold, to be selected, the pulses corresponding to thiswindow being separated from the other pulses.

Advantageously, this operation simultaneously considers, for eachdistribution, several different, predetermined statistical indicators,resulting in as many thresholds in each of the pixels considered, eachthreshold then corresponding to one of said statistical indicators.

Advantageously, this operation is carried out for all the pixels of adetector, but it may be performed on all or some of its pixels.

Advantageously, the lowest level threshold is located above theelectrical noise of the processing electronics.

Advantageously, the calibration is systematically carried out betweentwo measurements by the detection system

The invention will be better understood and other advantages will beclear on reading the following nonlimiting description and on examiningthe appended drawings among which:

FIG. 1 shows the layout of a part of a detector having a matrix ofsemiconductor pixels;

FIG. 2 shows a prior-art electronic calibration channel;

FIG. 3 shows the primary and secondary X-ray radiation in a detectorhaving a matrix of semiconductor pixels;

FIG. 4 shows the number of pulses detected as a function of the value ofthe threshold; and

FIG. 5 shows the electronic layout of a counting channel in a detectionsystem implementing the method according to the invention.

The calibration method according to the invention uses the X-raygenerator present in most X-ray detection systems. This generator isused at its nominal high voltage, i.e. between 15 and 160 kV. Aparticular feature of the response of semiconductor detectors, duringthe interaction of an incident photon with a detector pixel, is thatpart of the energy may be converted into lower-energy photons, which maythen deposit their energy in a neighboring pixel. For a detector of agiven volume, this effect is all the more apparent the more pixels thereare, which is the case in detectors that have an electrode divided intosmall pixels. The expression “small pixel” is understood to mean a pixelthe typical dimensions of which, defined by the distance over which theelectrodes repeat, are between a few tens of microns and a few hundredmicrons. When the interaction takes place near the edge of the pixel,the cloud of charges created there may also be shared between severaladjacent pixels. Thus numerous low-amplitude pulses are produced in eachpixel over a very wide energy spectrum. FIG. 3 illustrates this effect.The solid curve shows the spectrum S_(E) of the energy from the source,in this case a tungsten target, i.e. the incident radiation, the dottedcurve shows the spectrum S_(D) produced by a cadmium telluride (CdTe)semiconductor detector the anode of which is divided into small pixels,or elementary electrodes, such as those described above. The two spectraare shown as a function of the energy of the X-ray radiation in keV. Asmay be seen, this second spectra S_(D) contains low-energy spectrallines corresponding to the energies imparted during various interactionsin the detector and to the effect of charge carriers being sharedbetween adjacent pixels.

By counting the pulses emitted by each pixel through the effect of theincident radiation and by classing them according to their amplitudes inthe form of a histogram, an amplitude distribution of these pulses isobtained for each pixel the y-axis showing the amplitude and the x-axisshowing the number of pulses counted.

The incident radiation, produced by an X-ray generator, may beconsidered to be homogenous from the point of view of its energyspectrum and from the point of view of its intensity. Thus, when thedetector is illuminated by such radiation, each pixel is subjected toradiation having an identical energy spectrum. When the detector isconsidered to be of high quality it may be assumed that the amplitudedistribution of the counted pulses is similar for all the pixels.Similarly, it is expected that the distribution will be of comparableshape and area. This is notably due to the fact that there is no greatsensitivity difference between the pixels. The area under thisdistribution is also substantially the same for the all the pixels.

However, notably due to the electronic readout circuits differing fromone pixel to another, this amplitude distribution may undergo, from onepixel to another, a translation along the amplitude axis. Thus,interactions that impart the same energy in the detector may generatepulses of different amplitudes depending on the location of the pixelcollecting the charge carriers generated by this interaction. Without aparticular correction, the correspondence between the amplitude of apulse and the energy to which this pulse corresponds is not constantfrom one pixel to another. The expression “energy to which this pulsecorresponds” is understood to mean the energy deposited by theinteraction that generated this pulse.

It is necessary to correct this translation drift or effect, so that thecorrespondence between amplitude and energy is the same for all thepixels.

In the method according to the invention, such a correspondence isobtained by determining, for each distribution relating to each pixel, acriterion, for example a statistical indicator.

According to a preferred embodiment, this indicator is an Nthpercentile, i.e. N % of the counted pulses are below this value, the50th percentile corresponding to the median. Since the shape and thearea of the distribution are preserved from one pixel to another, such apercentile then enables a direct relationship between the amplitude andthe energy to be obtained. Thus, for each distribution, denoted D_(n),corresponding to each pixel, denoted n, the Nth percentile, denotedF_(n) ^(N %) corresponds to an amplitude A_(n) ^(N %). Without aparticular correction, the amplitudes A_(n) ^(N %) are different fromone pixel to another. However, since the pixels are subjected tohomogenous radiation, and the detector is of good quality, it isconsidered that the same energy E^(N) may be reasonably assigned to thispercentile for all the pixels considered.

Thus, by determining, for each pixel n, the amplitude A_(n) ^(N %) towhich the Nth percentile, denoted F_(n) ^(N %) n of the distribution Dncorresponds, a simple and direct correspondence between said amplitudeand the energy E^(N) corresponding to this percentile is obtained.

The calibration method according to the invention therefore comprisesthe following various steps:

Step 1: operating the X-ray generator at its nominal high voltage, thegenerator being placed so that the X-ray radiation seen by the detectoris homogenous, and preferably perpendicular to the receiving surface ofsaid detector. Advantageously an X-ray generator, notably the generatorinstalled in the device of the radiology equipment to which the detectorbelongs, is used. This allows all or some of the pixels to be repeatedlycalibrated, for example between to radiological examinations. Theamplitude distribution measured in each pixel has a shape near orsimilar to that shown in FIG. 3.

Step 2: counting, using the processing and calibration electronics, thepulses emitted by each pixel through the effect of the radiationproduced by the generator. By way of example, FIG. 5 shows a countingchannel. It comprises a detection pixel 10, amplification electronics20, processing electronics 21 for shaping the signal, N comparators 22that compare the energy of the pulses to N predetermined thresholds andN counters 23 each associated with one of the aforementionedcomparators.

During the calibration phase, it is moreover possible to use only asingle comparator, by varying the amplitude threshold beyond which thepulses are counted. It is thus possible to perform numerous countsC_(m), the detector being irradiated in the same way, and to count onlythe pulses having an amplitude that exceeds a threshold S_(m), thisthreshold being incremented between each count. Thus, if the thresholdS_(m) is increased between each count C_(m), in other words if S_(m) ishigher than S_(m-1) for two counts C_(m) and C_(m-1), the number ofpulses having an amplitude between S_(m) and S_(m-1) is the number ofpulses counted during the count C_(m) minus the number of pulses countedduring the count C_(m-1), the duration of the counts being assumed hereto be identical.

Alternatively, it is possible not to determine, during each measurement,counts, but count rates. An amplitude distribution is then establishedthat represents, for various amplitudes, not an occurrence but a rate ofoccurrence, i.e. a number of occurrences per unit time, or count rate.

Otherwise, other methods known to those skilled in the art may beimplemented to establish, for each pixel, a distribution of theamplitude of the pulses detected during exposure to an incident photonflux.

The data are then transmitted to a readout bus 24. The amplitudedistribution of the signal measured by each pixel is analyzed: it mayfor example be a histogram of counted pulses classed according to theiramplitude. As indicated above, it may for example be a histogramrepresenting the count rate of each pulse, the pulses also being classedaccording to their amplitude. When the detector is of high quality, itmay be assumed that, for each pixel, this distribution has a constantarea, implying that the sensitivity of the pixels does not vary greatly.In addition, when the incident radiation is homogenous, thisdistribution has a constant shape from one pixel to another. However,due to electronic drift, this distribution may be shifted in amplitude,this shift varying from pixel to pixel, and also in time. The thresholdis determined using a statistical indicator of this distribution. It maybe an Nth percentile of this distribution, the threshold correspondingto an amplitude below which N % of the area of the distribution isfound, i.e. N % of the pulses counted during this calibration phase. Itis possible to use other statistical indicators, such as the mean forexample, but Nth percentiles are the preferred indicators. Thus, bysubjecting all the pixels to a given exposure that is homogenous both inenergy and intensity, an indicator is determined for each pixel. Thisindicator may correspond, for the various pixels, to differentamplitudes, but, for all the pixels considered, it corresponds to agiven energy deposited in the detector. A correspondence betweenamplitude and energy is then established for each of the pixels.

According to a preferred embodiment of the invention, it is possible to“threshold” the pulses as function of this indicator, i.e. to retain,for example, only the pulses having an amplitude above this threshold.The various amplitude thresholds of the various pixels correspond to thesame energy. It is then understood that, by carrying out such amplitudethresholding on each of the pixels, thresholding to the same energylevel for all the pixels is achieved. By using various indicators,various amplitude thresholds result, each corresponding to variousenergy levels, these various energy levels being the same for all thepixels. By determining a number of thresholds, denoted Th(i), it is thenpossible to produce images corresponding to an energy band that containspulses the amplitude of which is between two of these thresholds Th(i)and Th(i+1).

FIG. 4 thus shows, for a given pixel, the number of pulses N_(D)received as a function of the energy E. In this figure, five thresholdsTh(i) are defined. The first threshold Th(1) may be defined according tothe noise acceptable for a given application. The counting noise N_(B),represented by a dashed line in FIG. 4, depends on the electrical noiseof the amplifier and on the proximity or nonproximity of the firstthreshold. It may be advantageous to subtract this noise so as not tofalsify the measurement in the first energy band.

Step 3: adjusting, using the processing and calibration electronics, thethreshold levels for each pixel so that each amplitude thresholdcorresponds to the same energy for all the pixels.

The number of pulses corresponding to a given threshold may be initiallycalibrated by various methods, the implementation of which may vary incomplexity, notably using perfectly calibrated sources. However, thepixel-by-pixel adjustment according to the invention using the X-raygenerator at its nominal voltage may be carried out regularly in orderto take account of electronic drift and instability of the semiconductormaterial. Such rapid calibration may be performed before each patient isscanned in the case of a medical scanner or between two sets of luggageduring inspection on a continuously operating detection line fordetecting suspicious products. Thus, it is possible to performcalibrations a little before, or a little after, the use of theradiology device. The aforementioned drift effects are then limited.

Another particularly important advantage of the invention is that thiscalibration may be carried out using the same generator as that usedduring the medical examinations. No additional X-ray source is required.

Such a method allows the response stability of semiconductor detectorsto be markedly improved, particularly for scanner imaging that requiresvery stable and very reproducible measurements.

1. A method of calibrating an X-ray detection system, said systemcomprising at least one X-ray generator and a detection array comprisinga matrix of detecting semiconductor pixels and processing andcalibration electronics, wherein the calibration method comprises:operating the X-ray generator at its nominal high voltage, the generatorbeing placed opposite the detector; counting, using the processing andcalibration electronics, the pulses emitted by each pixel through theeffect of the radiation produced by the generator; establishing, foreach pixel, an amplitude distribution of the counted pulses; applying,to each amplitude distribution, a statistical indicator so as toidentify a particular amplitude corresponding to an energy associatedwith said statistical indicator, said statistical indicator relating tosaid amplitude distribution being a percentile or the mean of saiddistribution; and adjusting, using the processing and calibrationelectronics, calibration parameters for each pixel, taking account ofthe energy-amplitude relationship thus established.
 2. The calibrationmethod as claimed in claim 1, wherein the statistical indicator relatingto said amplitude distribution is the median of said distribution. 3.The calibration method as claimed in claim 1, wherein the statisticalindicator is a combination of various percentiles.
 4. Calibration methodas claimed in , claim 1, wherein, in each pixel, the energy valuecorresponding to the amplitude of the signal corresponding to saidindicator is treated as being an energy threshold by the processing andcalibration electronics of the pixel.
 5. The calibration method asclaimed in claim 4, wherein the lowest level threshold is located abovethe electrical noise of the processing electronics.
 6. The calibrationmethod as claimed in claim 5, wherein the pulses having an amplitudebelow said threshold are not processed by the processing and calibrationelectronics of the pixel.
 7. The calibration method as claimed in claim4, wherein the processing and calibration electronics of the pixeldetermine at least one energy window, said window lying between a firstthreshold and a second threshold.
 8. The calibration method as claimedin claim 7, wherein the processing and calibration electronics of thepixel divide each distribution into as many energy windows.
 9. Thecalibration method as claimed in claim 8, wherein the operation ofdividing each distribution received by each pixel into as many energywindows is carried for the all the pixels in the detection matrix. 10.The calibration method as claimed in claim 8, wherein the operation ofdividing each distribution received by each pixel into as many energywindows is carried out for certain pixels of the detection matrix. 11.The calibration method as claimed in claims 1 wherein the calibrationmethod is systematically carried out between two measurements by thedetection system.